Ships may be vulnerable to attack from underwater swimmers. In order to defend from this threat anti-swimmer weapons such as the MK3A2 concussion grenade, small arms, .50 caliber machine guns, and ship sonar is used. Unfortunately conventional grenades have fixed time delay fuses (approx. 4 to 5 seconds) and will detonate at various depths depending on how long the grenade is held after activation, the height above the water the grenade is dropped from and how far the grenade is thrown. The MK3A2 has a limited lethal radius and is no longer in production. Although fragmentation hand grenades may be used, they are less effective in water than grenades that release pressure. Guns may be used to engage an attacker at long ranges in air but their projectiles only penetrate water to a depth of a few feet. Ship sonar powerful enough to disrupt a swimmer also affects underwater work in a large radius around the ship. Conventional grenades have inherent safety risk as well. Grenades may be dropped in the ship before thrown, harming personnel. There is a need for a safe, accurate grenade for defense from underwater attack.